The fine structure and biochemical characterization of synaptic junctions will illuminate the function of these complex units. Nerve cell structure and function is also reflected by analyzing differences between specific brain nuclei under control and experimental conditions. The relationship of the post-synaptic membrane to the submembranous post-synaptic density (PSD) will be studied using three major approaches: 1) the extraction of specific proteins of the synaptosomal plasma membrane fraction with various agents, to correlate ultrastructure of the resultant fraction with resulting protein patterns determined by gel electrophoresis, 2) high voltage electron microscopy to give a three-dimensional image of the PSD, demonstrating the relationship of the filaments to each other, their extent and the relationship of the PSD to the post-synaptic membrane, and 3) to correlate between the ultrastructure of developing synapses, isolated by cell fractionation techniques, and their protein composition, determined by gel electrophoresis. An investigation involving the ultrastructure of the ventromedial nucleus of the hypothalamus, a target of the hormone estrogen and implicated in the production of releasing factors, will elucidate the nature of these cells under control and experimental conditions.